A magnetic random access memory (MRAM) is a storage device utilizing a tunnel magneto-resistance (TMR) element or a magnetic tunnel junction (MTJ) element having a tunnel magneto-resistance (TMR) effect as a memory cell (a magnetic memory cell). An MRAM cell (a magnetic memory cell) is configured of two ferromagnetic thin films and a tunnel insulation film between the two ferromagnetic thin films. When the magnetic thin films on and under the tunnel insulation film are parallel in magnetic moment, a magnetic resistance attains a minimum value, whereas when the magnetic thin films are antiparallel in magnetic moment, a magnetic resistance attains a maximum value. This magnetoresistance effect is utilized to correlate large and small magnetic resistance values with logical values of “0” and “1”. In a data reading, a current is passed through a magnetic memory cell and information stored therein is read depending on the magnitude of an amount of the current.
The information stored in an MRAM cell holds a state of a magnetic moment until a magnetic field exceeding a threshold value level is applied in an opposite direction to change the direction of the magnetic moment. The stored information can thus be stored in a non-volatile manner substantially permanently.
Writing of storage information into an MRAM cell (a magnetic memory cell) is performed as follows: Write currents are passed through digit and bit lines orthogonal to each other. A combined magnetic field of the magnetic fields induced by the write currents changes a direction of a magnetic moment of a variable magnetoresistive element (a TMR element or an MTJ element) of the MRAM cell. The variable magnetoresistive element of the MRAM cell has two ferromagnetic films, one of which is utilized as a free layer and the other of which is utilized as a fixed layer. The fixed layer has its magnetic moment fixed in direction, whereas the free layer has its magnetic moment set in direction according to the storage information. A magnetic field of a sufficient magnitude to set the direction of the magnetic moment of the free layer is induced by the write currents on bit and digit lines to write data.
As high integration of MRAM advances, a cell is reduced in occupation area and bit and digit lines are also reduced in width. Accordingly, the bit and digit lines can pass smaller currents, resulting in a reduced intensity of an induced magnetic field. To compensate for such reduction in writing magnetic field due to miniaturization of cells, a clad interconnect structure is applied to bit and digit lines. Examples of the clad interconnect structure are described in patent literatures 1 and 2 (Japanese Patent Laying-open Nos. 2006-32762 and 2009-38221, respectively). In patent literatures 1 and 2, a high permeability film is provided to surround bit and digit lines in three directions, and a magnetic field is applied to a variable magnetoresistive element from the lines at their respective surfaces that are not surrounded by the high permeability film. The high permeability film is thus utilized as a magnetic shield film to confine a magnetic field for generating the magnetic field concentrately to induce a large magnetic field by the use of small write currents to set a direction of magnetization (or of a magnetic moment) of the variable magnetoresistive element.